A. Field of the Invention
The present invention relates to apparatus and methods to deter the leaching or displacement of contaminants in the ground by the flow of ground water, and in particular, to apparatus and methods to control and contain contaminants that exist in the ground at relatively shallow depths.
B. Problems in the Art
The following scenario is illustrative of the problems in the art relative to preventing relatively shallow underground contamination sites from leaching or displacing by ground water flow into human water supplies. An underground oil or septic tank develops a leak and contaminates the ground around it. Normally, there is a natural ground water gradient which causes ground water to flow in a direction through the ground. If left alone, the ground water flow would pass through the volume of contamination in the ground. The advective flow would tend to leach or displace at least some of the contamination and either form a "downstream" underground plume or flow of water/contaminant mixture. This puts at risk any downstream water well or water supply.
Unfortunately, there are reports that conclude there are tens of thousands of relatively shallow contamination sites around the United States that present the risk described above. One way to remedy the situation is to move any well that is possibly in the way of any down gradient leaching or displacement of contaminants or contaminated ground water. However, as can be appreciated, this is very costly and in some situations may not be possible because of limitations on where well placement can be or property ownership issues. Costs on the order of one-million dollars can be involved.
Therefore, solutions to these types of problems are needed. Attempts have been made to address these problems, to both contain and remediate contaminated sites (both the water and the soil). Physical barriers have been used. However, it is difficult over many years and the varied forces that can work under the ground to confidently assume that such barriers will last. Mechanical pumping of ground water out of the contaminated site has been tried. This can be costly and may not stop the ground water from carrying away contaminants. Hydraulic barriers have been attempted. Some utilize impervious layers that encapsulate the contaminated zone. As such, they are costly and difficult to build and maintain because many times they must completely surround the contaminants. No system has been satisfactory. Most either produce at best mediocre containment or control, and/or are expensive to build and operate. Some of the above-mentioned systems are sheet pile cutoff walls, low-permeability bentonite slurry walls, and hydraulic containment via ground water pumping.
There is therefore a need for improved apparatus and methods for control and containment of relatively shallow, in-ground contamination volumes. Attention has turned to solutions that try to control movement of the ground water.
For example, U.S. Pat. No. 4,288,174 to Laws discloses the placement of a sub-surface tile above or below a contaminated area. The patent speaks of attempting to slow the ground water flow by trying to level the water table. It recognizes that in most situations, the regional water table is tilted and has a gradient that causes movement of ground water. The attempt to bring the water table locally to a more level position by use of the tile, or wells, is intended to reduce the hydraulic gradient and thus reduce the flow of ground water through the in-ground contamination. This method does not stop local ground water flow, however.
The Laws patent teaches the reduction of flow through the contaminated volume by flattening locally the water table. It does so, however, only two-dimensionally, in the sense of relying on the flattening of the gradient of the ground water. There is a deficiency in that there remains the tendency of the dissolved contaminants to diffuse outward and away, even if there is a relatively flattened water table locally at the site of contamination. The Laws patent is incorporated by reference herein.
U.S. Pat. No. 4,580,925 to Matich et al. discloses a continuous impervious zone all around a contaminated zone. It also discloses the need for a cover over the zone. It discusses some of the problems in the art and is incorporated by reference herein. It does not address the problem of diffusion of contaminants, even if it tries to hydraulically slow down ground water flow caused by the gradient.
Others have looked to procedures such as natural attenuation, phytoremediation, and ecosystem restoration in conjunction with the problems discussed above. Phytoremediation attempts to use plants to process contaminated ground water. These attempts focus more on remediation and less on dealing with the risk of movement of contaminated ground water.
Cleanup of contaminated aquifers is notoriously difficult, and no single existing technology has proven entirely successful. Technical infeasibility is increasingly being invoked to justify leaving subsurface contaminants in place, especially within industrial areas that will most likely never to used for residential housing. These areas are sometimes called "brownfields". Nevertheless, existing regulations require that further contamination of the down-gradient aquifer be prevented as much as possible, which often necessitates some form of hydraulic control of ground water flow to prevent leaching of contaminants from the "source area."
Some of the reasons that existing technology has not solved the problems and deficiencies in the art are subtle but significant factors involving subsurface water.
Ground water movement is not necessarily completely predictable. The factors that affect it are dynamic. For example, there can be anisotropy within the aquifer. There may be substantial but temporary pulses of ground water caused by distant snow-melts. The water table tends to by dynamic, moving up and down. This affects the strata of soil, namely the unsaturated or vadose zone and the saturated zone. Thus merely looking at local leveling of the water table, such as by the Laws patent, or phytoremediation by plants alone is not a satisfactory more universal solution.
There is therefore room for improvement in the art regarding solutions to the problems with relatively shallow in-ground contamination zones. It is therefore a primary object of the present invention to provide an apparatus and method which controls and contains ground water flow in and around a contaminated zone in the ground which improves on or solves the problems and deficiencies in the art.
Other objects, features and advantages of the invention include:
1. long term hydraulic control; PA1 2. a passive system; PA1 3. economical regarding installation and on-going use; PA1 4. environmentally compatible; PA1 5. flexible in its application and uses; PA1 6. durable and long-lasting integrity PA1 7. low maintenance.
These and other objects, features and advantages of the invention will become more apparent with reference to the accompanying specification and claims.